354 PROCEEDINGS OF THE AMERICAN ACADEMY 



of geological succession. He was satisfied that the tracks in 

 the Potsdam Sandstone and the Trias, attributed to these an- 

 imals, were not made by them. The earliest remains of Tur- 

 tles were found in the Jura, and they are of the fresh- water 

 type. Now the order of position among Turtles places ma- 

 rine types lowest, next fresh-water, and last land Turtles. 

 There was a want of correspondence with the geological suc- 

 cession. From the descriptions of these specimens, and his 

 own recollection of them, however, Professor Agassiz was 

 satisfied that these fossils present a synthetic type, like the 

 Crinoids among the Radiata ; showing characteristics belong- 

 ing to several types. A similar instance he had found among 

 the living species of South American Turtles, from Brazil, 

 the Podoenemyds ; which, although belonging to the fresh- 

 water type, yet present certain characters in the formation of 

 the temporal region very similar to those of marine species, 

 constituting a true synthetic type. Professor Owen obtained 

 from the Chalk the next geological species which had been 

 discovered, and they belong to the Chelmians, with some 

 characters of the marine species, but not the general form, — 

 another synthetic type. Professor Agassiz did not regard 

 these forms as presenting true exceptions to the law of corre- 

 spondence between geological succession and the grade of de- 

 velopment in living species, but only as evidence of the exist- 

 ence in past ages, as well as the present, of what he had called 

 synthetic types. 



Professor Horsford announced a new and convenient method 

 of determining the value of saltpetre for the manufacture of 

 nitric acid, by ignition with sal ammoniac. 



Four liuudred and thirty-flftli meeting. 



January 2Sth, 1857. — Stated Meeting. 



The President in the chair. 



The Corresponding Secretary read a letter from Manuel J, 

 Johnson, of the Radcliffe Observatory, Oxford, accepting his 

 election as Foreign Honorary Member. 



